C#: No, you don’t get to use the “as” operator any more

http://badcodemonkey.wordpress.com/2011/04/29/no-you-dont-get-to-use-the-as-operator-any-more/

by Indyan • 15 years ago

22 41 15 years ago

badcodemonkey.wordpress.com

mendicant2• 15 years ago

Yes, null refs are bad.

But so is an InvalidCastException -- Which is what your new code could throw.

ondrasej• 15 years ago

I guess the point of the article is that when you're doing an invalid cast, you in fact want to get an InvalidCastException, not NullPointerException.

mendicant2• 15 years ago

That's part of it. But the author should articulate it.

There is also certain (specific) situations where the original code (e.NewItem as Customer).Save() would fail but ((Customer)e.NewItem).Save() wouldn't. That specific case is where the type of e.NewItem has an implicit cast to Customer (as would return null, but a (Customer) cast would work).

But if you're doing implicit casting you might be facing other issues.

kenjackson3• 15 years ago

But I do the null check all the time, and then skip the "Save", but have logic in the 'else' to do something like create an actual customer account.

The problem with the solution provided is that you get an exception, whether you can deal with it in the body or not. And unlike Lisp, you're going to unwind the stack at that point (or be in the catch). All in all, not happy with the proposed solution.

iramiller2• 15 years ago

Actually you don't want to ever use exception driven development. The use of the as operator is a simple metadata check. Throwing exceptions in .Net is expensive.

Just because I felt the need to check my own facts (Friday afternoon?) I wrote a simple console app with two classes Class1 and Class2 : Class1. Using a Stopwatch object I checked the elapsed ticks for an 'as' an 'is' and an invalid cast exception check.

'as' = 5 ticks

'is' = 4 ticks

'invalid cast' = 147990 ticks

Note this was just enough test to validate my assumption and that I never condone my developers taking an optimize first development approach... But there is still a right and a wrong way to do the job sometimes and exception driven development is not the right way.

lucisferre• 15 years ago

Yeah I no argument that it's good to know what your code does but what you choose to do is context specific, why would throwing InvalidCastException be better than NullRef, neither tell me why the failure happened.

On a somewhat unrelated note why is this trivial C# 101 lesson on the bloody front-page. I've got to imagine maybe 1 out of 10 readers care and far fewer didn't already know this.

jbrechtel3• 15 years ago

InvalidCastException include the class name, doesn't it? That makes it order of magnitude easier to track down where it came from. The NullRef just tells you that it MAY have been that the 'as' failed, or it could have been a bona-fide NullRef.

k2xl• 15 years ago

strange... ecma script allows it...

snprbob86• 15 years ago

Many language designers, including many on Microsoft's Managed Language teams, consider null reference and invalid cast exceptions to be huge blunders.

The "Nice" language shows that it's pretty easy to statically avoid this: http://nice.sourceforge.net/safety.html

The sad fact is that the Java and .NET base class libraries were designed before (1) generics and (2) a generic "Nullable" (aka "Option") type were available. Anyone designing a statically typed environment today should certainly include both and design a compiler to enforce their usage and safety.

Groxx3• 15 years ago

... but since 2.0 .NET can do all the things listed in that Nice document. There are glaring gaps in the core libraries, which tick me off to no end, but that has nothing to do with the language.

masklinn6• 15 years ago

Nice is a language, not a VM, so I'm guessing you're talking about C#.

In which case... uh... no. Of Nice's "advanced safety features" it only got generics (in 2.0) and internal iterators (in 3.0, via LINQ methods)

* It's a single-dispatch language

* Its instanceof (`is`) is the same shit as Java's (and the compiler is just as stupid)

* Object types are always nullable, it's not optional and it's not opt-out

* Its arrays are covariant (covariant arrays were added to the CLR for java compatibility, and C# got them from there because it was "free")

ssp3• 15 years ago

The main problem with non-nullable types is that the static analysis involved doesn't work for object fields., so you can't write code like this:

        if (object.some_field != null)
               object.some_field.foo();

because the compiler is unable to prove that it's correct. Instead you have to do a clumsy workaround like this:

    SomeType some_local;

    some_local = object.some_field;
    if (some_local != null)
        some_local.foo();

Groxx3• 15 years ago

That seems like it would be a trivial thing to do in an early compilation step... why is the workaround necessary?

masklinn6• 15 years ago

> The main problem with non-nullable types is that the static analysis involved doesn't work for object fields., so you can't write code like this

You shouldn't get to write code like that anyway. It's C. It's crap.

Here's what you write:

    object.some_field.notNull(f => f.foo())

and that's only valid if some_field is of type Nullable<whatever>.

munificent2• 15 years ago

If you have to check it for null then, by definition, some_field here isn't a nullable type. If it was, you'd just do:

    object.some_field.foo();

Am I missing something here?

bartwe• 15 years ago

Nonnullable (static) class fields and constructors don't mix however, which is why you don't see it as a language addition so far.

Nonnullable local variables and function parameters however could easily work within the current c# language.

noblethrasher3• 15 years ago

I would make the public version of Save a static method on an abstract class (or a static method in a utility class) and avoid the whole worry over null. It's only a bit more work up front.

    abstract class Foo
    {
        public static Result Save(Foo foo)
        {
            if(foo != null)
               foo._Save();
            else
            {
                //Do Stuff...
            }

        }

        protected abstract Result _Save();
    }

Then use like so,

    foreach(Foo s in xs)
    {
        Foo.Save(s); //or s.Save() if Save() is an extension method.
    }

danparsonson• 15 years ago

Or use an interface, then anyone making changes further down the line could leave the 'Save()' calling code alone entirely.

noblethrasher3• 15 years ago

That would work too but an abstract class let's you keep the _save method private so that client code can't possibly call that instance method on null.

Personally I think that now that we have extension methods, all public methods should be extension methods on interfaces or abstract classes. This basically gives you something like the pattern matching in ML and Haskell.

jbrechtel3• 15 years ago

This misses the author's point. In his example the bug you want to detect is that the referenced object in question is not a Customer (or a Foo in your example).

In your else block you can't report anymore information than the NullPointerException in the author's original example reported.

If you still use an 'as' operator at the callsite then you've lost all useful information. If you do a cast then you'll at least get an exception with WHAT it was you were trying to cast.

noblethrasher3• 15 years ago

I don’t think that I’ve missed the author's point; but any time I see an if statement and polymorphic code used near each other, I question whether or not the programmer understands OOP. His example is not exactly using polymorphic code, but he is basically dispatching based on the runtime type.

We know that a method that accepts parameters that are reference types should check to see if they are null. Since instance methods cannot check their first parameter (the this pointer), we make it impossible for outside code to call instance methods. Now callers never have to worry about the runtime type of an object or whether or not it’s null.

If you’re going to circumvent the type system with explicit casts then, of course, you’re going to get invalid cast exceptions, but that’s for the client code to deal with.

InclinedPlane• 15 years ago

((e.NewItem as Customer) ?? Customer.NullObject).Save();

Edit: The idea being, you have a sub-class of Customer which implements the Null Object pattern, which you instantiate through a static member on Customer or a Property method or what-have-you. The great thing about this is that you can tailor the behavior in this situation to whatever you want. Want to throw an exception? Sure, throw whatever exception you want from CustomerNullObject.Save(). Want to log an error and otherwise silently fail? No prob, easy peasy.

singular2• 15 years ago

There are subtleties to it too - http://blogs.msdn.com/b/ericlippert/archive/2009/10/08/what-...

:-)

jasonkester• 15 years ago

It's best if you think about the reason that "as" was added to C#. It's so you can do this:

  Customer c = person as Customer;
  if (c != null)
  {
    // do something
  }
  else
  {
    // not a customer.  must be just a regular person. do something else
  }

instead of this:

  if (person is Customer)
  {
    Customer c = (Customer)person;
    // do something
  }
  else
  {
    // not a customer.  must be just a regular person. do something else
  }

... and that's it. It's just somebody's idea of a little cleaner syntax, not having to cast twice to get where you want to be.

In that light, the author's second example is closest to correct. It's just missing the "else" bit to handle the case where the base object you get passed is not be the exact type you're looking for.

singular2• 15 years ago

EDIT: Actually, yes, ahem, I see - if e.NewItem is actually null you're going to get confused between what the problem actually is here, and that might not be repro. Perhaps I posted a little too soon... but assuming you check for that separately, the point stands :)

I have to disagree here - I prefer the initial code example. It's cleaner, and as soon as you get to the line of code in the debugger you'll know exactly what's up regardless of the on-the-surface-misleading null exception. So what if it's initially confusing?

In any case both examples are wrong, and I don't think that's made clear enough - you should be actively looking to check for exceptions, not passively accepting that they might happen.

As an aside, I've always found it cleaner to say:-

    var cust = e.NewItem as Customer;
    if(cust == null)
        throw new BlahException("blah blah blah");

    cust.Save();

Compared to, say:-

    if(!(e.NewItem is Customer))
        throw new BlahException("blah blah blah");

    var cust = (Customer)e.NewItem;
    cust.Save();

(Note that in the second case that a thread could jump in and spoil your day anyway, not to mention that .NewItem could be some crazy property which randomly decides to return null in the second case, and you'd get the same old nasty exception, as mentioned in other posts)

So I think the 'as' operator has value in this alone as handy sugar.

Obviously the real problem is that null exceptions happen at all, or that anything actually returns null, and by God is the Option discriminated union in F# a joy (union of Some/None) after dealing with all that shizzle.

scottdw2• 15 years ago

Actually... he misses a big use case (stolen from haskell)... Using extension methods (which accept null receivers) and "as" you can get pretty readable straight line code... something like:

    name = (obj as Customer).LiftName() ?? "Guy Incognito"

Interestingly, built in support for this (w/o the need to write a "LiftName" extension method) almost made it into VB 2008, but it ended up getting cut.

tomjen3• 15 years ago

I tend to use it because it makes it reading from left to right feel right instead of jumping back and forth.

And really, don't give me null objects unless I specifically say I can handle them.

C#: No, you don’t get to use the “as” operator any more

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